This invention relates generally to pulse-width modulators for converting an analog signal into a pulse-width modulated signal and to driving units for actuators to be used in optical-type recording and/or reproducing apparatus, and particularly relates to a driving unit which forces an output transistor to turn on and off at a high frequency and controls its on-state period with the aid of the pulse-width modulator.
In the optical-type recording and/or reproducing apparatus, as is well known, the tracking control and the focusing control are made on the very small spot of focused light, and thus an actuator is needed to move optical components such as an object lens and so on which constitute the optical head, thereby moving the light spot.
In recent years, as small-sized and power-saving apparatus have been requested, a driving method using a high-efficiency pulse-width modulation system had been employed.
FIG. 12 shows the arrangement of a conventional pulse-width modulator and driving unit which employs it to drive the actuator, and FIG. 13 is a waveform diagram of waveforms at each portion of the arrangement of FIG. 12. Referring to FIG. 12, an input signal S1 is supplied to a comparator 1 and to an absolute value circuit 2. THe comparator 1 discriminates the polarity of the input signal S1 to produce a digital signal S2 which is, as shown in FIG. 13, level "H" when the signal S1 is positive (+) relative to the reference level "0" and is level "L" when it is negative (-). This signal S2 is supplied to an invertor 3 to produce therefrom a signal S3 which is the inversion of the signal S2.
The absolute value circuit 2 is used to produce the absolute value of the input signal S1. As shown in FIG. 13 at S4, the absolute value circuit produces the signal S1 when the signal S1 is positive, and it produces the inversion of the signal S1 relative to the "0" level, or the absolute value signal S4. This signal S4 is supplied to the non-inverting input terminal of a comparator 4. To the inverting input terminal thereof is supplied a triangular wave signal S5 of a high frequency (several tens of kHz to several hundreds of kHz) which rises from the "0" level, and which is produced from a triangular wave generation circuit 5. These signals are compared with each other by this comparator 4 to produce a signal S6 which is level "H" when the signal S4 is larger than the signal S5, and "L" when the signal is smaller than the signal S5. This signal S6 is a pulse-width modulated signal of which the "H" period increases in proportion to the level of the signal S4. This signal S6 is supplied to NAND gates 6 and 7. THe signal S2 is supplied to the other input of the NAND gate 6, and the signal S3 to the other input of the NAND gate 7. Thus, the NAND gate 6 produces a signal S7 which, as shown in FIG. 13, becomes the inversion of the signal S6 when the signal S2 is .cent.H" , and becomes "H" when the signal S2 is "L". The NAND gate 7 produces a signal S8 which, as shown in FIG. 13, is "H" when the signal S2 is "H", and is the inversion of the signal S6 when the signal S2 is "L".
The signals S2, S3, S7 and S8 are supplied to a switching circuit 8. In the switching circuit 8, the emitters of PNP transistors Q1 and Q2 are connected to a power supply Vcc, while the emitters of NPN transistors Q3 and Q4 are grounded. The collectors of the transistors Q1 and Q3 are connected to a terminal 18a of an actuator 18, and the collectors of the transistors Q2 and Q4 to a terminal 18b thereof. A flywheel diode D1 has its anode grounded and its cathode connected to the terminal 18a. A flywheel diode D2 has its anode grounded and its cathode connected to the terminal 18b. The signal S7 is supplied to the base of the transistor Q1 through a resistor R1, the signal S8 is supplied to the base of the transistor Q2 through a resistor R2, the signal S2 is supplied to the base of the transistor Q4 through a resistor R3, and the signal S2 is supplied to the base of the transistor Q4 through a resistor R4. For the convenience of explanation, it is assumed that these transistors make the ideal operation in which the on-resistance is 0 .OMEGA. and the off-resistance is .infin..
When the input signal S1 is positive, as described above the signal S3 is "L", and the signal S8 is "H", so that the transistors Q2 and Q3 are in the off-state. Also, the signal S2 is "H" so that the transistor Q4 is in the on-state, thus, the terminal 18b being grounded. This results in the equivalent circuit as shown in FIG. 14(a). In this circuit, a drive current Ia proportional to the "L" period of signal S7 which corresponds to the on-period of the transistor Q1 is flowed from the terminal 18a to the terminal 18b.
On the contrary, when the signal S1 is negative, the signal S2 is "L", and the signal S7 is "H", so that the transistors Q1 and Q4 are turned off. Also, since the signal S3 is "H", the transistor Q3 is in the on-state, thus the terminal 18a being grounded. This results int he equivalent circuit as shown in FIG. 14(b). Thus, as in FIG. 14a the drive current Ia is proportional to the "L" period of the signal S8 which corresponds to the on-period of the transistor Q2 and flows from the terminal 18b to the terminal 18a.
Therefore, if the current flowing from the terminal 18a to the terminal 18b is a positive current, the drive current Ia is proportional to the input signal S1 and flows to the actuator 18 as shown in FIG. 15.
While int he above description, the output signal S5 from the triangular wave generation circuit 5 rises from the "0" level, the case in which the triangular wave S5 rises from an off-set level above the "0" level will be described below.
FIG. 16 is a waveform diagram of waveforms at each portions of the arrangement of FIG. 12 in the case where the signal S5 has an off-set voltage Vof1 in the positive direction. Referring to FIG. 16, when the signal S4 is lower than the voltage Vof1, the signal S6 is always "L" and the two pulse-width modulated signals S7 and S8 are "H". Thus, the insensitive zone in which no pulse occurs is caused, so that the transistors Q1 and Q2 are in the off-state, thus no current Ia flows. Therefore, the current Ia with respect to the signal S1 has the insensitive zone, Vdz as shown in FIG. 17.
FIG. 18 is a waveform diagram of waveforms at each portions of the arrangement of FIG. 12 in the case where the signal S5 has an off-set voltage Vof2 in the negative direction. Referring to this figure, even when the signal S4 reaches "0" level, the signal S4 is larger than the minimum level of the signal S5. At this time, the signal S6 becomes "H" only for the period, .DELTA.T corresponding to the off-set voltage Vof2. Thus, when the signal S1 is changed form the positive to negative or from negative to positive level, the pulse-width modulated signal S7 or S8 may have such pulse width ("L" period ) as to discontinuously skip not via "0" as .DELTA.T .fwdarw. -.DELTA.T or -.DELTA.T .fwdarw..DELTA.T (it is assumed that the pulse width of "L" level of signal S7 is represented by +, and that the pulse width of "L" level of signal S8 is denoted by -) (hereinafter, this skip characteristic is called the discontinuous characteristic). In addition, the "L" period of the signal S7 or S8 relative to the signal S1 is longer than in the case of FIG. 13. Thus, the drive current Ia vs signal S1 characteristic is as shown in FIG. 19, that is, the current Ia is discontinuously changed at around "0" of signal S1 as +.DELTA.I1.fwdarw.-.DELTA.I1.
Moreover, in FIG. 12, the comparator 4 has a response time in which the triangular wave signal S5 and the absolute value signal S4 are compared with each other, this response time being another factor which causes the discontinuous characteristic and the insensitive zone. If the relation between the rise time, tr and fall time tf of this comparator is tr &gt; tf, the "L" period of the signal S6 becomes short, thus causing the insensitive zone in the pulse-width modulated signals S7 and S8 as in FIG. 16 and also the insensitive zone int he characteristic of the drive current with respect to the input signal S1. On the contrary, if tf &gt; tr, the "L" period of the signal S6 becomes long, so that the discontinuous characteristic is caused in the relation of the input signal to the pulse-width modulated signals S7 and S8 or to the drive current.
As described above, the conventional pulse-width modulator has the drawback that the insensitive zone and the discontinuous characteristic are easy to be caused by the off-set of the triangular wave signal and the response time of the comparator, making the pulse-width modulated signal erroneous. The driving unit using this modulator is similarly easy to produce the insensitive zone and the discontinuous characteristic, so that it is impossible to obtain a correct drive current proportional tot he input signal.
Moreover, when this conventional driving unit is incorporated in the controller for the driving of the actuator of the optical recording and/or reproducing apparatus, occurrence of the insensitive zone will result in the fact that even if the input signal is changed in the sensitive zone, the current Ia flowing into the actuator is not changed. Consequently, the gain is reduced so that the controller cannot maintain high-precision control. In addition, when the discontinuous characteristic appears, the current to the actuator is greatly changed with the change of the input signal around "0" level so as to increase the equivalent gain, thus making the control system unstable.
Further, the triangular wave signal rising from the "0" level, or the off-set is easy to occur as shown in FIGS. 16 and 18. Also, since the frequency of the triangular wave signal is normally as high as several tens of kHz to several hundreds of kHz, it is difficult to realize a comparator which can operate at so high a frequency that the period can be neglected.
Therefore, the insensitive zone and the discontinuous characteristic becomes great, making it further difficult to operate the pulse-width modulator and the driving unit using it with high precision.